BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

help needed

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Senior | Next Rank: 100 Posts
Posts: 54
Joined: Wed Jul 20, 2011 12:36 pm
Thanked: 2 times

help needed

by sumasajja » Thu Aug 11, 2011 3:10 am
a rectangular game board is composed of identical squares arranged in rectangular array of r rows and r+1 columns.the r rows are numbered from one through r, and the r+1 columns are numbered from 1 through r+1.if r>10 which of the following represents no. of squares on the board that are neither in the fourth row nor in the seventh column?
a]r^2-r
b]r^2-1
c]r^2
d]r^2+1
e]r^2+r
Top
Source: — Problem Solving |
Legendary Member
Posts: 1448
Joined: Tue May 17, 2011 9:55 am
Location: India
Thanked: 375 times
Followed by:53 members

by Frankenstein » Thu Aug 11, 2011 3:31 am
Hi,
'r' rows and 'r+1' columns of squares = r(r+1) squares.
4th row(for that matter any row) has (r+1) squares
7th column(in fact any column) has r squares
They have 1 square in common.
So, number of squares in 4th row or 7th column = (r+1)+r-1 = 2r
So, remaining squares = r(r+1) - 2r = r^2 - r

Hence, A
Cheers!

Things are not what they appear to be... nor are they otherwise
Top
Senior | Next Rank: 100 Posts
Posts: 54
Joined: Wed Jul 20, 2011 12:36 pm
Thanked: 2 times

by sumasajja » Thu Aug 11, 2011 3:35 am
i think r rows and r+1 columns will have a total of (r-1)*(r) squares
please explain why isnt it so
Top
Legendary Member
Posts: 1448
Joined: Tue May 17, 2011 9:55 am
Location: India
Thanked: 375 times
Followed by:53 members

by Frankenstein » Thu Aug 11, 2011 3:39 am
sumasajja wrote:i think r rows and r+1 columns will have a total of (r-1)*(r) squares
please explain why isnt it so
Hi,
That is when you are drawing r horizontal lines and (r+1) vertical lines. But, in this case, we are given that the squares are placed in r rows and (r+1) columns. So, it will be r(r+1) squares. If you are still confused draw a figure with r=3 or so. It should help you understand better.
For example, consider a chess board, it has 9 vertical lines and 9 horizontal lines, but the squares are placed in 8 rows and 8 columns. Same here.
Last edited by Frankenstein on Thu Aug 11, 2011 3:40 am, edited 1 time in total.
Cheers!

Things are not what they appear to be... nor are they otherwise
Top
User avatar
Master | Next Rank: 500 Posts
Posts: 324
Joined: Mon Jul 05, 2010 6:44 am
Location: London
Thanked: 70 times
Followed by:3 members

by kmittal82 » Thu Aug 11, 2011 3:39 am
sumasajja wrote:i think r rows and r+1 columns will have a total of (r-1)*(r) squares
please explain why isnt it so
If this were true, then a board of 1 row and 2 columns would mean 0 squares
Top
Senior | Next Rank: 100 Posts
Posts: 54
Joined: Wed Jul 20, 2011 12:36 pm
Thanked: 2 times

by sumasajja » Thu Aug 11, 2011 4:06 am
kmittal82 wrote:
sumasajja wrote:i think r rows and r+1 columns will have a total of (r-1)*(r) squares
please explain why isnt it so
If this were true, then a board of 1 row and 2 columns would mean 0 squares
ya now i understand,thanks
Top
Legendary Member
Posts: 2789
Joined: Tue Jul 26, 2011 12:19 am
Location: Chennai, India
Thanked: 206 times
Followed by:43 members
GMAT Score:640

by GmatKiss » Thu Aug 11, 2011 5:56 am
IMO: A

Given, r > 10

consider r=11 and r+1 = 12
total squares = r(r+1)=132
4th row and 7th column = 11+12 = 23

total remaining squares = 109 + 1(common squarea) = 110

(r^2)-r = 110
(11)^2-11 = 110
Top
Post new topic Post Reply
• Page 1 of 1